The invention relates to the electrochemical conversion of reactants to products by utilizing alternating electrical current and semiconductor electrodes. More particularly, the invention relates to electrolytic processes and the electrocatalysis of redox reactions.
Electrochemical methods of manufacture are becoming ever increasingly important to the chemical industry due to their great ecological acceptability, potential for energy conservation and the resultant cost reductions possible. A great deal of effort has been applied to the hardware employed in such methods.
A major element of the hardware is the electrode itself. In general, requirements of an effective electrode include cost, stability, conductivity and electrocatalytic activity. An electrode cost within the range of commercial feasibility is desirable. Ordinarily, the electrode must be relatively stable toward its own oxidation or reduction by reactants, products and/or intermediates in an electrochemical cell. The conductivity of both the substrate and surface layers of the electrode is important from the viewpoint of minimizing voltage losses associated with the ohmic drop across the surface and the formation of surface insulating layers. The electrocatalytic activity of the electrode for a selected reaction contributes toward lowering the potential and raising product selectivity in an electrochemical cell.
Only a few materials may effectively constitute an electrode, particularly as an anode, because of the susceptibility of most substances to intense corrosive conditions. Among the effective electrode materials are graphite, nickel, lead, lead alloy, platinum or platinized titanium. Electrodes of this type have limited application because of such disadvantages as lack of dimensional stability high cost, low chemical activity, contamination of the electrolyte, contamination of the cathode deposit, sensitivity to impurities, lack of selectivity for the desired reaction, and ineffective generation of current densities or high overpotentials when employed in cells overpotentials typically refer to the excess electrical potential over the theoretical potential at which the desired product is discharged at the electrode surface). Effective current densities of electrodes employed in a large number of commercial electrochemical cells are ordinarily greater than about 50 milliamps/square centimeter (mA/cm.sup.2).
Electrochemical cells operate on direct current electricity (DC) at relatively high currents and relatively low voltages. Economic incentives for electrochemical cells operating on alternating current electricity (AC) are high, provided rectification costs of AC to DC can be minimized. For example, the rectification and power conditioning equipment for a water electrolysis plant may be at least about 15 percent of the total hardware or capital costs of the plant and typically nearly 40 percent of the cost of the cell itself. Although a number of compounds have been electrosynthesized using alternating current with conventional electrodes, electrosynthetic processes have demonstrated substantial loss of current efficiency when AC frequencies are increased. For example, the current efficiency in a cell containing platinum flag electrodes for producing propylene oxide with alternating current decreases from 22 percent at a frequency of 1 Hz to 2 percent at a frequency of 40 Hz. (See Alkire and Tsai, J. Electrochemical Soc., Vol. 129, p. 1157 (1982)).
Many attempts and proposals have been disclosed to overcome some of the problems associated with electrochemical cells, none of which seems to have accomplished an optimization of the desirable characteristics for such cells. Presently, there is interest in the development of rectifiers of alternating current electricity and numerous rectification means have been employed.
However, a need still exists for electrochemical cells employing inexpensive electrodes capable of rectifying alternating current electricity that have suitable electrocatalytic properties for selected reactions in such cells.
Accordingly, it is an objective of the invention to provide an apparatus which utilizes relatively inexpensive materials for rectification of alternating current electricity and which is nevertheless stable during electrochemical processes and, additionally, is preferably electrocatalytic to a number of reactions, such as oxidation or reduction of organic molecules, water, and the like.
It is a further objective of the invention to provide an electrochemical apparatus comprising an electrode capable of generating significant direct electricity current from alternating current when immersed in an electrochemical solution.
A further object still is to provide an electrochemical process for converting reactants to products with a cell containing an electrode capable of rectifying alternating current electricity.
A still further object is to provide an electrochemical cell employing an electrode capable of rectifying alternating current electricity and, more particularly, to provide a cell employing such an electrode capable of generating significant current densities.
These and other objects and advantages of the invention will become apparent from the following description.